Dynamometers have been used many years in the vehicle manufacturing industry for testing vehicles in place as distinguished from road testing. Vehicle dynamometers are used in various testing operations. One principal use is that of measuring the torque and horsepower output of a vehicle. Another principal use is that of simulating the road load forces and the inertia forces acting upon a vehicle which it encounters during actual operation of the vehicle on a roadway. In both applications, the dynamometer must be capable of measuring roll torque with a high degree of accuracy.
In recent years, the regulation of vehicle emissions has become increasingly stringent for the purpose of protecting the environment and for fuel economy. In the United States, the Environmental Protection Agency has established vehicle standards which require highly sophisticated chassis roll dynamometers for road simulation in the testing of vehicles. In this capacity, the dynamometer must simulate the forces acting on the driving tires of a vehicle travelling on a roadway. In this simulation function, the vehicle must be operated on the rolls of the dynamometer and exhibit the speed, torque, acceleration and distance travelled over extended time periods as it would if actually driven on a roadway in the same manner.
A typical chassis roll dynamometer comprises at least one dynamometer roll which is rotatably mounted in a stationary frame and which supports one driving wheel of a vehicle to be tested. The roll is typically of large diameter, several times that of the vehicle wheel. There is driving engagement between the vehicle wheel and the roll by reason of the tractional forces of the wheel such that the vehicle wheel may drive the roll or vice-versa, depending upon the test being conducted. The dynamometer system also comprises apparatus for simulating road load forces acting upon the vehicle wheel and for simulating inertia forces acting upon the vehicle during acceleration and deceleration. The road load forces include rolling friction and windage. The apparatus for simulating road load forces comprises a motor coupled with the roll through a roll shaft and a controller for controlling energization of the motor in accordance with the simulation or testing being conducted. In some systems, a flywheel is coupled with the roll shaft for simulating inertia. The motor of the dynamometer system is sometimes referred to as a power exchange unit because it may be operated to apply power to or absorb power from the vehicle wheel through the roll. The motor is operated as a torque generating or absorbing device and may be either a DC or an AC motor, for example, with a suitable electronic controller which includes a computer.
In a dynamometer system of the type described above, it is common practice to provide instrumentation for the measurement of the torque output of the vehicle wheel and the velocity of the wheel. The torque is measured by a torque transducer connected in the drive train of the dynamometer for providing a roll torque signal to the controller. Wheel speed is measured by a shaft encoder suitably connected with the roll shaft of the dynamometer for supplying a velocity signal to the controller.
A longstanding problem in achieving a high degree of accuracy in vehicle simulation and testing is that of providing accurate compensation for torque measurement errors caused by friction in the roll shaft bearings due to applied load and viscosity friction. It has been a common practice in chassis roll dynamometers to install the torque transducer in the driven shaft between a shaft bearing and the motor. In this arrangement, the torque transducer does not produce an accurate measurement of the torque applied to or absorbed from the roll because it is influenced by the bearing torque. In the prior art, there have been attempts to provide compensation for the measurement errors arising from bearing friction. Such prior art has disadvantages in that the torque measurement errors are not precisely compensated or eliminated and a complex and costly system is required. Also, changes in oil viscosity, applied load and tractive effort forces will influence bearing friction torques.
A dynamometer system which provides compensation for certain friction and windage losses is described in the D'Angelo et al. U.S. Pat. No. 4,327,578 granted May 4, 1982. This patent describes a chassis roll dynamometer with a roll assembly which is connected by a shaft with a motor and a flywheel mounted on the shaft between the motor and the roll assembly. A torque transducer between the motor and the flywheel provides a torque signal to a system controller and a speed transducer on the shaft provides a speed signal to the controller. The system utilizes an equation to determine the actual force output of the vehicle which expresses a functional relationship between the force reading from the torque transducer, the measured control speed and the mechanical inertia outside the torque transducer loop. The patent states that because the force outside the torque loop is accounted for, the torque transducer is free to be placed between the flywheel and the DC motor and also permits the torque transducer be placed next to the rollers or even directly on the vehicle itself. It is noted in the patent that the frictional and windage parasitic losses are subtracted from the torque sensor signal when the signal is conditioned. In particular, the patent refers to the friction and windage compensation for parasitic losses occurring within the DC motor by adding certain factors to the calculated force signal to obtain a compensated force signal which is applied to the power converter to control the armature current to the DC motor. According to this prior art it is known to compensate a force or torque measurement signal to for certain errors arising from friction and windage in the system.
A chassis roll dynamometer which is provided with friction compensated bearings on the roll shaft is described in SAE Paper 930391 entitled "Large Roll Chassis Dynamometer with AC Flux Vector PEU and Friction-Compensated Bearings" dated Mar. 1-5, 1993. In this dynamometer, the roll bearing friction is compensated mechanically by providing a bearing race carrier between the inner and outer races which is driven by a motor at the same speed as the roll shaft. The motorized bearing is designed to cancel the effect of the bearing friction which would otherwise produce a torque which would be sensed by the torque transducer and be indistinguishable from the torque applied by the vehicle tires.
A general object of this invention is to provide an improved chassis roll dynamometer which overcomes certain disadvantages of the prior art.